1. Technical Field
The present invention relates to an arrangement of data connections associated with communication services such as telephone, meeting and game services which require as short a data transmission delay as possible.
2. Discussion of Related Art
A cable television system is most conventionally a distribution network resembling a tree, at the root of which lies the main amplifier of the operator controlling the network, i.e. the so-called head-end. The main amplifier may more generally be called a central configuration. The distribution lines branch from it in a tree-like fashion towards the data terminal equipment of the subscribers, of which there may be hundreds of thousands under one central configuration. In order to compensate the weakening of the signal and to reduce interferences, the distribution lines comprise distributing amplifiers, repeaters and other devices known per se.
Lately, plans have been proposed to change the cable television systems from one-way distribution networks to two-way data transmission networks. In this case, the direction of data transmission from the central configuration to the data terminal equipment is generally called downstream (DS), and the reverse direction upstream (US). In addition to a high-capacity downstream main channel, the system includes additional channels; these include at least an upstream channel, through which the data terminal equipment may transfer data to the direction of the central configuration, and a relatively low-capacity downstream control channel, though which the central configuration controls the use of upstream connections. The control channel may consist of cyclically recurrent fields which the central configuration multiplexes with the digital video picture or another signal transmitted in the main channel by utilizing its frame structure. Such a control channel is a so-called in-band control channel. In another embodiment, the control channel is situated on an own frequency band, thus being a so-called out-of-band control channel.
The present invention may be applied to digital video systems known in themselves, i.e. DAVIC (Digital Audio Visual Council) and DVB (Digital Video Broadcasting). The system specifications significant for the invention are found in the publications "DAVIC 1.0 specification part 08; Lower layer protocols and physical interfaces, December 1995", "DAVIC 1.0 corrigenda part 0.8; Lower layer protocols and physical interfaces, Edited version after New York meeting, Rev. 2.1, June 1996", "DAVIC 1.1 specification part 08; Lower layer protocols and physical interfaces Rev. 3.3", and "ETSI draft specification prETS 300 800; Digital Video Broadcasting (DVB); DVB interaction channel for cable TV distribution system (CATV), TM 1640 Rev. 4, June 1996". The cable TV system disclosed in the publications may be based on coaxial cables, or at least partly on optical fibres; in the latter case, it is also called a HFC (Hybrid Fibre Coax) network.
FIG. 1 illustrates a proposition included in the publication prETS 300 800 for allocating frequencies in a DVB system. The dimensions on the horizontal frequency axis are indicative, and the vertical axis only shows which signals are directed towards the data terminal equipment from the central configuration (DS, upwards in the figure), and which extend to the reverse direction (US, downwards in the figure). The frequency range 100 extends from about 50 MHz to almost 900 MHZ, and it is typically divided into channels 102 of 6-8 MHz, of which only three are shown for clarity. Each channel contains one QAM (Quadrature Amplitude Modulation) modulated signal which may include, for example, one or more digital video signals in a MPEG-TS format (Motion Picture Experts Group--Transport Stream) or other data requiring a high transfer capacity. The frequency range 103 extends from 70 MHz to 130 MHz, and it contains channels 104, which are 1 or 2 MHz wide, each transferring a QPSK (Quadrature Phase Shift Keying) modulated control channel. For clarity, of these only three are shown in the figure. The frequency range 105 extends from 300 MHz to 862 MHz, and its contents correspond to the frequency range 103. The frequency range 106 of the figure, extending from 5 MHz to 65 MHz, is reserved for upstream connections, and it contains channels 107, of which only three are shown in the figure, and which are 200 KHz, 1 MHz or 2 MHz wide. The QPSK modulation is also intended to be used in upstream connections. The new specifications of the DAVIC system are, however, also prepared to use the QAM modulation in channels of low capacity.
A transmission especially in a channel of the DAVIC system containing data about the use of one or more upstream channels, consists in accordance with FIG. 2 of SL-ESF frames 108 (Signalling Link Extended Superframe). The length of one SL-ESF is 4632 bits, and it is divided into 24 frames of 193 bits. In FIG. 2, the frames are numbered from 1 to 24, and additionally, an enlarged frame is described with the reference number 109. Each frame starts with a so-called overhead bit 110, which is followed by a payload 111 of 192 bits. The significance of the overload bit depends on which frame of the SL-ESF is observed. In flames 4, 8, 12, 16, 20 and 24 included in the SL-ESF, the value of the overhead bit is a fixed framing bit, i.e. a so-called F bit. Correspondingly, the overhead bits of the frames 2, 6, 10, 14, 18 and 22 are so-called C bits, which, when placed successively, form a CRC checksum describing the bit contents of the previous SL-ESF. In every other frame, beginning from frame 1, the overhead bit is an M bit, i.e. part of a so-called M counter indicating the numbering and timing of the slots of an upstream channel controlled in this control channel.
Communications in each upstream channel is divided into slots 112, as described in FIG. 3. The central configuration determines the use of the slots so that part of the slots may be used for ranging aiming at the measurement and compensation of transfer delays, part are conventionally freely available for data terminal equipment (so-called contention-based slots), part has been determined for the use of data terminal equipment having made a reservation in accordance with a specific reserve inventory (so-called reservation slots), and part has an operation time schedule distributing a certain regular data transmission capacity for the use of one connection (so-called contentionless-based slots). In the DAVIC system, the central configuration transmits data concerning the use of eight upstream channels in one downstream channel.
The communication of downstream and upstream channels is synchronized so that each downstream SL-ESF, the M bits M1, M5 and M9 (in SL-ESF, the sequence numbers of these bits from the beginning of SL-ESF are 0, 1544 and 3088) correspond to the so-called slot position references. If the bit rate of a downstream channel is 1,544 Mbit/s, the period of two successive slot position references accommodate three upstream slots, i.e. the temporal length of one downstream SL-ESF is the same as the added temporal length of nine upstream slots. If the bit rate of an upstream channel is 256 kbit/s, one upstream slot is temporally as long as the time from one slot position reference over the next to the next one so that the temporal length of one downstream SL-ESF is the same as the added temporal length of one and a half upstream slots. If the bit rate of an upstream channel is 3,088 Mbit/s, the period between two slot position references accommodates six upstream slots, i.e. the temporal length of one downstream SL-ESF is the same as the added temporal length of 18 upstream slots.
For the identification of downstream SF-ESFs and upstream slots, they are numbered cyclically. The numbering of SL-ESFs runs from 9 to N, where N is the size of the cycle, i.e. the largest sequence number used for SL-ESF. The cycles mean that the SL-ESF number 0 is followed by SL-ESF number 1, which is followed by SL-ESF number 2, and so on, until SL-ESF number N is again followed by SL-ESF number 0, and the numbering starts all over. If the bit rate of a downstream channel is 3,088 Mbps, two sequential SL-ESFs are always given the same sequence number, i.e. the sequence number changes only at the place of every other SL-ESF. The M bits M10-M1 of each SL-ESF form a 10-bit register where M10 is the most significant bit and M1 the least significant, and the value of which indicates the sequence number of the SL-ESF. As the register includes 10 bits, the largest possible value for the number N is 2.sup.10. The data terminal equipment maintain the numbering of upstream slots, which is synchronized with the numbering of SL-ESFs. If, for example, the bit rate of 1,544 Mbps is used both in downstream and upstream, the slots from 0 to 8 correspond to the SL-ESF number 0 and the slots from 9 to 17 correspond to the SL-ESF number 1, and so forth.
A data terminal equipment, to which a certain slot of an upstream channel has been assigned, transmits a burst of 64 bytes during the slot in question. A burst 113 is shown in more detail in FIG. 3. At the beginning of the burst there is a synchronization period 114 of four bytes, which is called Unique Word and which corresponds to the hexa number sequence CC CC CC 0D. It is followed by a payload 115 of 53 bytes, which most commonly is an ATM cell, and a six-byte Reed-Solomon code 116 is calculated from its contents. At the end of the burst there is a guard period 117 of one byte.
The upstream slots are grouped as shown in FIG. 3, the groups including 3, 9 or 18 slots depending on the bit rate (256 kbps, 1,544 Mbps or 3,088 Mbps). In the case of FIG. 3, each group contains 9 slots, i.e. it is an upstream channel of 1,544 Mbps. The data terminal equipment for which a certain regular data transmission capacity is assigned in accordance with the contentionless principle, may, for example, receive one slot from each group. In the case of FIG. 3, a certain data terminal equipment, which is denoted with A, has received the fourth slot in every group. The data terminal equipment B makes it with half a smaller data transmission capacity so that it receives one slot (in the Fig. the sixth slot) from every other group. The data transmission need of the data equipment terminal C is further half of the data transmission need of B so that it receives one slot from every fourth group.
If the data terminal equipment requires more data transmission capacity than the data terminal equipment A, it is assigned several slots from each group. In accordance with the cyclical indication of slots defined in the DAVIC system, the slots received by a certain data terminal equipment are defined so that they are situated equidistantly in a string of upstream slots. In order for this precondition to be in harmony with the fact that, for facilitating the indication arrangements, the slots assigned for a certain data terminal equipment have to be situated in successive slot groups at same places in relation to the group's start and end, only certain multifolds of slots are possible for achieving a larger data transmission capacity. Depending on the bit rate of the upstream channel, the permitted multifolds are 1 and 3 (with bit rate of 256 kbps); 1, 3 and 9 (with the rate of 1,544 Mbps), or 1, 3, 9 and 18 (with the rate of 3,088 Mbps). The assignment of contentionless-based slots is realized so that the Connect-message transmitted to the data terminal equipment by the central configuration includes a so-called Cyclic.sub.-- Assignment record of six bytes in length, which contains three fields of two bytes long, i.e. a start field, a distance field, and an end field. The central configuration uses these fields to inform in which slot the data terminal equipment may start its transmission, what is the mutual distance between the slots assigned to it, and in which slot the data terminal equipment has to end the transmission.
The data transmission rate equalled by one slot in each slot group may be called the root bit rate (RBR). When the bit rate of the upstream channel is 256 kbps, the root bit rate is 64 kbps. If the bit rate of the upstream channel is 1,544 Mbps or 3,088 Mbps, the root bit rate is 128 kbps. The possible data transmission rates corresponding to different slot arrangements permitted for one data terminal equipment are, for example, 1,152 Mbps, 384 kbps and 128 kbps in an upstream channel of 1,544 Mbps, and bit rates smaller than those above so that the next bit rate is always half the former.
When trying to apply the said state-of-the-art arrangement for arranging a two-way data transmission in a cable television system to carry out delay-critical services, one is confronted with certain problems. Here, the delay-critical services refer to so-called communicative services in which messages in the nature of questions and answers are exchanged between the sender and the receiver, the questioner wishing to receive the answer as soon as possible. The most common delay-critical services are telephone connections and various teleconference services, but also some multi-player games set similar demands for the shortness of delays. While the tele services are diversifying, new services are generated all the time, part of which will always be delay-critical. The delay causes both the slowing down of the communication and the disturbing echoing of the sent data from the receiver back to the sender.
A digital telephone connection in accordance with the publication ITU-T Recommendation G.711 is viewed as an example; this connection conveys digital speech signals generated on a sampling frequency of 8 kHz and with an 8-bit A/D conversion so that the bit rate required for data transmission is 64 kbps. It has to be noted that data to be transferred is also generated with the same bit rate; if ATM cells with a payload of 48 bytes (384 bits) are used for data transmission, filling an ATM cell with the rate of 64 kbps takes 6 milliseconds. In order to optimally use the capacity of the channel, the equipment does not transmit the ATM cell forward before it is full, so that merely filling the ATM cell causes a delay of 6 ms to the signal. In the publication "Lee, Kyoo J.: Signal Delay Requirement for IEEE 802.14 protocol to support voice application, IEEE 802.14, Jan. 10, 1996, IEEE 802.14-96/011" it is proposed that the reciprocating delay for the telephone service should be less than 3,4 ms so it is obvious that the data transmission system based on ATM cells described above is not able to provide delay-critical services in a desired way.